1. The binding of sodium dodecyl sulphate to proteins by equilibrium dialysis was investigated. 2. Most of the proteins studied bound 90-100% of their weight of sodium dodecyl sulphate. 3. The glycoproteins studied bound 70-100% of their weight of sodium dodecyl sulphate, calculated in terms of the polypeptide moiety of the molecule. 4. Proteins not containing S.S groups bound about 140% of their weight of sodium dodecyl sulphate. 5. Reduction of four proteins containing S.S groups caused a rise in sodium dodecyl sulphate binding to 140% of the weight of protein. 6. The apparent micellar molecular weights of the protein-sodium dodecyl sulphate complexes were measured by the dye-solubilization method; they were all found to have approximately the same micellar molecular weight (34000-41000) irrespective of the molecular weight of the protein to which they were attached.